The present invention relates to a method and an arrangement for sound-suppression in wheels in accordance.
A person who is in the passenger compartment in a motor vehicle often experiences that he/she is in a noisy environment. The noise level, however, varies depending on the size and manufacture of the vehicle one is sitting in. It is also obvious that noise increases when the vehicle accelerates. A common inference is therefore that the noise principally comes from the engine. However, it is so that a relatively large amount of the noise inside the passenger compartment originates from road noise. In modern cars a great deal of effort has been applied to minimize the engine noise which penetrates into the passenger compartment and also road noise but there still remains a lot when it is a question of the latter. Minimizing the noise from the road and the wheels is moreover becoming more important as the engine noise decreases because the noise from the road and wheels in this case, relatively seen, forms a larger part of the total noise in the passenger compartment.
The noise which is usually attributed to the wheels of the vehicle comprises partly the external noise which is produced when a wheel meets the road surface and partly the sound which is produced in the air cavity inside the wheel because of a standing wave being excited which is based on the natural frequency (resonance frequency) of the wheel. Thus there occurs what can be described as a resonance tone inside the wheel. The natural frequency of the wheel is determined by the circumference in such a way that the circumference of the internal air cavity is equal to one wave length. The bigger the wheel, i.e. the bigger the circumference, the longer the wave length and therewith the lower the natural frequency. A 15" wheel has a natural frequency of approximately 230 Hz. The frequency interval which is of interest in this connection is from approximately 250 Hz down to just under 100 Hz, where the latter corresponds to a truck wheel. The external noise which belongs to the meeting of the tire with the road surface depends e.g. on the appearance of the tire, the pattern of the tread and the material of the tire, as well as the road surface. This noise is airborne. The noise which occurs inside the wheel as a result of resonance vibration is to a certain degree airborne but to the greatest part it is a sound body, i.e. the sound propagates via the wheel suspension and further into the passenger compartment of the car.
The object of the present invention is to suppress there so-called natural frequency sounds in wheels which originate in the air cavity inside the wheel.
Devices for suppressing sound inside wheels are known from EP-A-0 663 306 and JP-A-07 052 616. From these documents it is known in the prior art to suppress sound inside wheel cavities by quite simply placing a sound-absorbing material there. The effect of this must be considered to be extremely limited. In EP-A-0 663 306 is mentioned e.g. that the invention is to suppress noise which originates from the natural vibrations in the wheel cavity and, as an example, natural frequencies from 150 to 250 Hz are mentioned. In the last mentioned document it is also shown how resonators can be arranged in connection to the sound absorbers inside the tire. These resonators are, however, intended to absorb sounds other than natural frequency sound, as their design is such that their dimensions limit sound suppression to frequencies of the order of 1000 Hz, which greatly exceed the natural frequency of a vehicle wheel. As mentioned, the natural frequency is determined by the circumference of the wheel. A natural frequency of 1000 Hz would correspond to a wheel with a diameter of approximately 11 cm.
The use of resonators for sound suppression in wheels is in itself also known from EP-B-0 041 127 and EP-A-0 665 529. In the first of these documents the resonator is on a sound shield which is placed on the outside of the wheel. In the second of these documents resonators are placed in the wheel housing. Both these documents therefore show sound suppression of external airborne sound originating from the wheel's contact with the ground. Neither of these documents treat the problem of body-transmitted sound depending on the natural vibrations inside the wheel.
Finally, in order to illustrate the prior art, JP-A-487 803 is mentioned, from which document it is known to half the wave length and thereby double the resonance frequency inside a wheel by producing an intermediate wall inside the wheel. The resulting double frequency is not experienced to be equally noisy as the original frequency. Here, however, it is not a question of sound suppression in a strict sense.